Jagadese J. Vittal

Storage performance of LiFePO4 nanoplates

The morphology of electrode materials is addressed as a key factor controlling rapid lithium storage in anisotropic systems such as LiFePO4. In view of this, we have synthesized nanoplates of LiFePO4 with a uniform coating of a 5 nm thick amorphous carbon layer by the solvothermal method and investigated their electrochemical storage behavior. The obtained nanoplates are well characterized by XRPD, SEM, HRTEM and XPS techniques. The thickness along the b-axis is found to be 30–40 nm; such a morphology favors short diffusion lengths for Li+ ions, while the external conductive carbon coating provides connectivity for facile electron diffusion, resulting in high rate performances. Increase in the size of the nanoplates results in poor lithium storage performance. The storage performance of nanoplates is compared with that of mesoporous nanoparticles of LiFePO4 with non-uniform carbon coating. This paper thus describes the advantages of thinner nanoplates for high rate storage performances of battery electrode materials.

Morphology controlled synthesis of LiFePO4/C nanoplates for Li-ion batteries

Lithium iron phosphate, LiFePO4 (LFP), is considered to be a potential cathode material for lithium-ion batteries but its rate performance is significantly restricted by sluggish kinetics of electrons and lithium ions. A simple solvothermal method has been described in this article to synthesize carbon coated LFP (LFP/C) nanoplates with varying thickness from 20 to 500 nm by using different iron precursors. The influence of solvents on the morphology of the LFP in the solvothermal synthesis is also investigated. A uniform carbon coverage at the surfaces has been achieved by a selective chelating carbonising source, D-gluconic acid lactone. The smallest dimension of the nanoplates has been found to be the b-axis where the Li+ ion diffuses quickly. The overall capacity and rate performance have, in general, been found to increase with the decrease of thickness of the nanoplates. Hierarchical LFP/C with ∼30 nm thickness shows the best electrochemical performance of 167 mA h g−1, followed by spindle (<20 nm thickness but aggregated, 121 mA h g−1), plates (200–300 nm thickness, 110 mA h g−1) and diamond shaped LFP/C (300–500 nm thickness, 82 mA h g−1) at a current rate of 17mA g−1 (0.1C rate). The spindle shaped LFP/C shows unexpected electrochemical performance since the nanoplates are heavily agglomerated in the bulk which prevents access for the liquid electrolyte, as well as additive Super P carbon, between neighbouring nanoplates during the fabrication of the composite electrodes. Hence, only the peripheral plates of the spindle are actively involved in the insertion/extraction of Li+, while the core of the spindle shaped LFP/C is almost inactive, resulting in moderate storage behaviour.

Thermal Conversion of a Helical Coil into a Three-Dimensional Chiral Framework.

Thermal dehydration results in the solid-state supramolecular conversion of the helical coordination polymer [{[Cu(sala)](2)(H(2)O)}(n)] into the chiral three-dimensional covalent open network [{Cu(sala)}(n)] (shown schematically). X-ray crystallography reveals that hydrogen bonding plays a key role in this process. H(2)sala=N-(2-hydroxybenzyl)-L-alanine

Trinuclear Heterobimetallic Ni2Ln complexes [L2Ni2Ln][ClO4] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er; LH3 = (S)P[N(Me)N═CH−C6H3-2-OH-3-OMe]3): From Simple Paramagnetic Complexes to Single-Molecule Magnet Behavior

The reaction of LH3 with Ni(ClO4)(2).6H 2O and lanthanide salts in a 2:2:1 ratio in the presence of triethylamine leads to the formation of the trinuclear complexes [L2Ni2Ln][ClO4] (Ln=La (2), Ce (3), Pr (4), Nd (5), Sm (6), Eu (7), Gd (8), Tb (9), Dy (10), Ho (11) and Er (12) and L: (S)P[N(Me)NCH-C6H3-2-O-3-OMe]3). The cationic portion of these complexes consists of three metal ions that are arranged in a linear manner. The two terminal nickel(II) ions are coordinated by imino and phenolate oxygen atoms (3N, 3O), whereas the central lanthanide ion is bound to the phenolate and methoxy oxygen atoms (12O). The Ni-Ni separations in these complexes range from 6.84 to 6.48 A. The Ni-Ni, Ni-Ln and Ln-O phenolate bond distances in 2-12 show a gradual reduction proceeding from 2 to 12 in accordance with lanthanide contraction. Whereas all of the compounds (2-12) are paramagnetic systems, 8 displays a remarkable ST=(11)/2 ground state induced by an intramolecular Ni. . .Gd ferromagnetic interaction, and 10 is a new mixed metal 3d/4f single-molecule magnet generated by the high-spin ground state of the complex and the magnetic anisotropy brought by the dysprosium(III) metal ion.

Synthesis, structure, and magnetism of heterobimetallic trinuclear complexes {[L2Co2Ln][X]} [Ln = Eu, X = Cl; Ln = Tb, Dy, Ho, X = NO3; LH3 = (S)P[N(Me)N=CH-C6H3-2-OH-3-OMe]3]: A 3d-4f family of single-molecule magnets.

Sequential reaction of LH3 (LH3 = (S)P[N(Me)N=CH-C6H3-2-OH-3-OMe]3) with Co(OAc)2 x 4 H2O followed by reaction with lanthanide salts afforded trinuclear heterobimetalllic compounds {[L2Co2Ln][X]} [Ln = Eu (1), X = Cl; Ln = Tb (2), Dy (3), Ho (4), X = NO3] in excellent yields. These compounds retain their integrity in solution as determined by electrospray ionization mass spectrometry studies. The molecular structures of 1-4 were confirmed by a single-crystal X-ray structural study and reveal that these are isostructural. In all of the compounds, the three metal ions are arranged in a perfectly linear manner and are held together by two trianionic ligands, L3-. The two terminal Co(II) ions contain a facial coordination environment (3N, 3O) comprising three imino nitrogen atoms and three phenolate oxygen atoms. The coordination geometry about the cobalt atom is severely distorted. An all-oxygen coordination environment (12O) is present around the central lanthanide ion, which is present in a distorted icosahedral geometry. The coordination sphere around the lanthanide ion is achieved by utilizing three phenolate oxygen atoms and three methoxy oxygen atoms of each ligand. In all of these trinuclear complexes (1-4), the Co-Ln distances are around 3.3 A, while the Co-Co distances range from 6.54 to 6.60 A. The screw-type coordination mode imposed by the ligand induces chirality in the molecular structure, although all of the complexes crystallize as racemates. Magnetic properties of 1-4 have been studied in detail using dc and ac susceptibility measurements. Dynamic measurements reveal that 2-4 display a single-molecule magnet behavior, while the Co2Eu (1) analogue does not show any out-of-phase ac susceptibility.

Topochemical Conversion of a Hydrogen-Bonded Three-Dimensional Network into a Covalently Bonded Framework

Thermal dehydration promotes the topochemical conversion of the hydrogen-bonded dimeric complex [{Zn(sala)(H2 O)2 }2 ]⋅2 H2 O (H2 (sala)=N-(2-hydroxybenzyl)-L-alanine) to generate covalent [{Zn(sala)}n ]. X-ray crystallography reveals that hydrogen bonding plays a key role in this process (see the partial structure on the right).

Stacking of double bonds for photochemical [2+2] cycloaddition reactions in the solid state

trans-1,2-Bis(4-pyridyl)ethylene (bpe), containing a C=C bond, has been extensively studied for solid-state photochemical [2+2] cycloaddition reactions, in discrete molecular and metal complexes, hydrogen bonded and coordination polymeric structures. The challenges in orienting a pair or more of bpe molecules in the solid-state using crystal engineering principles, and their photochemical behaviour and implications, based on Schmidts postulates, are discussed.

Coordination networks of Ag(I) and N,N′- bis(3-pyridinecarboxamide)-1,6-hexane: structures and anion exchange

A bidentate ligand N,N′- bis(3-pyridinecarboxamide)-1,6-hexane (L) and its silver complexes have been synthesized and characterized by single crystal X-ray diffraction. Reaction of AgClO4 and L in H2O/EtOH gives rise to a coordination polymer [Ag2L3OH][ClO4]·2.5H2O. The X-ray crystal structure of the compound shows honeycomb-like networks in which four-coordinated Ag ions are linked to three-coordinated Ag ions via three ligands L. The coordination of the long ligands L to the Ag ions creates tube-like structures and the tubes of adjacent honeycomb layers are interlocked, leading to an interpenetrating network. The compound [AgL][ClO4], synthesized from CH3OH, is composed of twisted zigzag coordination polymers in which ligands L are linked by Ag ions. Ligand L displays two different conformations A and B within a single strain of polymer. The two conformers differ in the orientation of the two pyridyl-groups which are arranged periodically in the polymer in the sequence ABBABBA. The polymer chains assemble into 2-D undulating sheets via amide hydrogen bonds. Reaction between AgNO3/AgCF3SO3 and L leads to polymeric compounds [AgL][NO3] and [AgL][CF3SO3]. The compounds are composed of coordination polymers in zigzag conformation and the polymer chains assemble into undulating sheets via inter-chain hydrogen bonds. The inter-sheet Ag–Ag distances of the compounds are in the order [AgL][CF3SO3] > [AgL][ClO4] > [AgL][NO3]. The anion exchange properties of the compounds are monitored by using X-ray powder diffraction, infrared spectroscopy and elemental analysis. Our results show that the anions in [AgL][NO3] and [AgL][CF3SO3] can be totally replaced with ClO4−. However, the exchange is not reversible. In additional, inter-conversion between [AgL][NO3] and [AgL][CF3SO3] by anion exchange is shown to be unfeasible. Anion selectivity could be due to the different hydration energy of the anions and the structural reorganization involved in the conversion.

Distortional Supramolecular Isomers of Polyrotaxane Coordination Polymers: Photoreactivity and Sensing of Nitro Compounds†

Distortional isomers, or bond-stretch isomers, differ only in the length of one or more bonds, which is due to crystallographic disorder in most cases. The term distortional isomerism is introduced to describe the structures of polyrotaxane 2D coordination polymers (CPs) that differ only by the relative positions in the neighboring entangled axles. A large ring and a long spacer ligand in 2D CPs yielded four different supramolecular isomers, of which two have an entangled polyrotaxane structure. One pair of C=C bonds in the spacer ligand is well-aligned in one isomer and undergoes [2+2] cycloaddition reaction, whereas the other isomer is photoinert. They also have different sensing efficiency for several aromatic nitro compounds. However, both isomers show selective PL quenching for the Bradys reagent. Structurally similar supramolecular isomers with different photochemical reactivity and sensing abilities appear to be unprecedented.

Lithium storage in a metal organic framework with diamondoid topology – a case study on metal formates

In this manuscript, a systematic investigation on the electrochemical performance of as-synthesized metal organic framework (MOF) Zn3(HCOO)6 with diamondoid structure for the Li storage using conversion reaction at low potential is described. Nearly an invariable capacity of 560 mAh g−1 (9.6 moles of Li) was obtained up to 60 cycles at 60 mA g−1 within the voltage range 0.005–3.0 V. The regeneration of the MOF during the cycling and the improved cyclability are evidenced from the electrochemical results along with ex situ PXRD, FTIR and TEM studies. The electrochemical cycling suggests that metal formate frameworks react reversibly with Li through conversion reaction. The matrix involved during the cycling was lithium formate rather than the typical Li2O and this is well supported by the ex situ FTIR results. The thermodynamic feasibility to transform the lithium formate to transition metal formate is more highly favored than from Li2O and this is further confirmed by reacting lithium formate with respective transition metal nitrates. The reversible formation or regeneration of FOR1 MOF plays a vital role in attaining the superior Li storage performance. Ultimately, the observation of improved storage performance and good cycling stability of Co3(HCOO)6 and Zn1.5Co1.5(HCOO)6, and the overall simple and eco-friendly synthesis method demonstrates that robust, thermally stable MOFs are a prospective class of electrode materials for Li ion batteries (LIBs).